1. Field of the Invention
This invention pertains generally to the field of paperboard packaging cartons, and particularly to cartons which are adapted to be used in microwave ovens in connection with the cooking of food contained within the cartons.
2. Description of the Prior Art
In the microwave cooking of various types of foods, it is often desirable to be able to shield a portion of the food from microwave radiation, while directing the radiation to other portions of the food. For some specific types of foods, it may actually be desirable to cook the food primarily by conduction heating, which is accomplished by shielding the major portion of the food product within a carton from microwave radiation, while utilizing a layer of microwave absorber in close proximity or contact with the food which heats the food as it absorbs radiation. A common example of a type of food product which should be heated in this way is frozen pizza--since it is very desirable that the bottom crust be heated thoroughly by conduction without overcooking the pizza sauce lying on top of the crust.
A number of package constructions have been devised which attempt to satisfy these objectives. For example, a laminate of metal foil and paper placed over the product within a paperboard carton has been used to shield the contents from radiation from the top and sides. Similarly, the food material has been packaged within a metal foil pouch having a cut-out window which allows the microwaves to be directed toward a predetermined portion of the food product. Other structures have used a layer of metal foil adhered to the surface of the carton itself to shield a portion of the food from the microwaves. While such known structures usually perform satisfactorily if the product is heated with the package intact, problems often arise if the consumer attempts to reheat food within the package after it has been opened.
Typically, once the integral structure of known shielded packages has been disrupted, there often exist separated portions of the metal foil which are split away from adjacent portions of the foil, or are layered adjacent to other foil portions not in direct electrical continuity therewith. In addition, sharp, crinkled edges in the foil may be formed during opening, and these sharp edges tend to develop high electric field intensities during application of microwaves. The result of such disruptions in the foil shield is the creation of very high potential differences between various points within the package as the package is subjected to microwave radiation. The electrical potentials can become sufficiently high that arcing takes place across the gaps between adjacent foil portions; since these gaps may contain paperboard or even portions of the food product, charring of the paperboard or food is common; on occasion, such arcing becomes so severe that the paperboard ignites.